This invention relates to a torque sensor detecting a shaft torque applied to a rotary shaft as change of magnetic flux density (i.e., strength of magnetic field).
As a conventional technique, Japanese Patent Application Laid-open No. 2003-149062 (FIG. 13) discloses a torque sensor.
According to this torque sensor, as shown in FIG. 13, a first shaft 100 and a second shaft 110 are coaxially connected by a torsion bar 120. A multipolar magnet 130 is fixed to the first shaft 100 (or one end of the torsion bar 120). One set of magnetic yokes 140, fixed to the second shaft 110 (or the other end of the torsion bar 120), is disposed in a magnetic field generated from the multipolar magnet 130. The magnetic yokes 140 are opposed to each other via an air gap in the axial direction. A magnetic sensor 150 detects the density of magnetic flux generated in the air gap.
Each of the magnetic yokes 140 has a plurality of claw poles disposed at equal pitches in the circumferential direction. The claw poles of respective magnetic yokes 140 are spaced with a predetermined air gap in the axial direction and are angularly offset from each other by a half pitch in the circumferential direction.
The magnetic yokes 140 and the multipolar magnet 130 are disposed in such a manner that the center of each claw pole of the magnetic yokes 140 agrees with a magnetized boundary line (i.e., a boundary between N-pole and S-pole), to obtain a neutral point where the output (voltage) of magnetic sensor 150 becomes zero in the condition that the torsion bar 120 causes no torsion (i.e., when no torque is applied to the torsion bar 120).
The above-described prior art document discloses a fixing portion 160 used for positioning the set of magnetic yokes 140. However, the detailed arrangement of fixing portion 160 is not disclosed and accordingly it is not apparent how the magnetic yokes 140 are positioned (especially, in the circumferential direction). Furthermore, it is not clear how the condition of magnetic yokes 140 positioned by the fixing portion 160 is retained.